When transient DNA strand breaks formed by DNA topoisomerase I fail to religate, the topoisomerase becomes irreversibly attached to the 3' DNA end via a tyrosyl linkage. This linkage must then be cleaved by tyrosyI-DNA phosphodiesterase (Tdp1) in order to allow repair of the break. Human deficiency in Tdp1 has been implicated in hereditary spinocerebellar ataxia with axonal neuropathy (SCAN1), the clinical features of which are similar to those of other ataxias associated with oxidative stress. These similarities, combined with the finding that Tdp1 also removes glycolate moieties from 3' ends of free radical-mediated DNA strand breaks, suggest that Tdp1 may be involved in repair of oxidative DNA damage, and that failure to repair such damage may lead to neuronal dysfunction in SCAN1. In order to test these hypotheses, cytotoxicity as well as repair of 3'-phosphoglycolate-terminated DNA double-strand breaks will be examined in Tdp1-deficient lymphoblastoid cells, derived from SCAN1 patients. Oxidative DNA damage to telomeric DNA, as well as telomere structure and function, will be compared in normal and Tdp1-deficient cells. Both conventional and conditional Tdp1 knockout mice will be generated and extensively characterized. This will include an assessment for the development of ataxia or other behavioral dysfunctions as well as for neuronal apoptosis both in the presence and absence of oxidative stress. Tumor incidence will be determined at a number of sites in both untreated animals and those exposed to oxidative stress, and the effects of Tdp1 deficiency on tumor progression will be evaluated in mice that are predisposed to the development of premalignant prostate neoplasia. In addition, mice will be monitored for signs of accelerated aging. These studies are intended to clarify the mechanism by which Tdp1 deficiency leads to SCAN1, and may also help elucidate the role of oxidative stress in various other neurological disorders, cancer and aging.